A range extender in a hybrid electric vehicle consists of a small internal combustion engine which drives an alternator to produce electrical energy. This electrical energy supplements the electrical energy produced by a battery or battery system to power the electric motor powering the drive system of the vehicle. Range extenders are used to extend the limited range of purely electric vehicles. Because current battery technology cannot provide the required electrical energy to give a pure electric vehicle sufficient range, a hybrid electric vehicle having a range extender offers a compromise between an internal combustion powered vehicle and a pure electric vehicle. This compromise improves vehicle performance and extends the vehicle's range while keeping emissions minimal.
One approach toward achieving the objectives of a range extender is described by A. Kalberlas in an article "Electric Hybrid Drive Systems For Passenger Cars And Taxis", ELECTRIC VEHICLE DESIGN AND DEVELOPMENT, published by the SAE in February 1991. This article teaches the use of a small internal combustion engine operated at fixed operating points within its engine speed/torque map. The fixed operating points are selected so that the engine is operated to deliver maximum energy compromising both efficiency and emissions. A corresponding approach is disclosed by Michel Elienne in U.S. Pat. No. 4,187,436.
The disadvantage of these range extender systems is that the internal combustion engines are operated at a fixed speed causing the electrical energy generated by the associated alternator to be greater than electrical energy demanded by the operator most of the time during each trip. The excess energy is used to charge the batteries. Consequently, a considerable portion of the electrical energy generated by the range extenders of the prior art is converted to chemical energy in the batteries. Because the efficiency of converting electrical energy into chemical energy, and vice versa, is low, the electrical energy generated by the alternator charging the batteries is an inefficient use of this electrical energy. Another disadvantage of the range extenders taught by the prior art is that the batteries may be charged to a nearly fully charged state. This approach is not desirable because charging the batteries by electricity from a conventional source of electrical energy, such as a household source of energy, as opposed to charging the batteries by the range extender minimizes the use of gasoline which, in turn, minimizes the emissions of the internal combustion engine. Additionally, the life of the batteries may be shortened due to the high charging rate of the prior art range extender systems.
The invention is a method for controlling a range extender, to supply the electric drive system of the vehicle with electric power corresponding to the average electric power being consumed in a preceding time period, to control the range extenders delivered power so that a majority of regenerative braking energy is used to charge the battery, to control the rate of charge of the battery and to prevent the internal combustion engine from overheating.